A widely used image processing technique is to convert a visible image into electronic data by encoding the brightness of adjacent small areas of the visible image. Such electronic encoding is advantageous for manipulation, transmission and storage of images. It is known to reconvert electronic data into visible images by means of a so-called "scanner system" whereby a finely focussed beam of light is rapidly scanned across a light sensitive medium in a succession of abutting raster lines, whilst modulating the intensity of light so as to reproduce the required image densities, based on the electronic signals.
Lasers, especially those using argon, krypton, helium-neon or helium-cadmium mixtures as the gas lasing media, have been used as sources of high intensity light for this imaging technique. However, the lasers all suffer the disadvantage of requiring an additional, complex device to modulate the intensity of light emitted, and to a greater or lesser extent, from large physical bulk, mechanical fragility and expense of manufacture.
Semiconductor laser diodes are potentially highly suitable as light sources for scanner systems in that their light output can be directly modulated by the electrical signal input, and that they are very compact and physically durable.
However, at present the only commercially available laser diode devices to have acceptably long operational life-times, and be capable of cheap manufacture, are those emitting light in the near-infrared (NIR) portion of the spectrum, from 750 to 1500 nm. Accordingly, in order to utilize laser diode scanner systems for imaging purposes it is necessary to provide a recording medium which is sensitive to light in the NIR range.
It is known to spectrally sensitise photographic silver halide emulsions to near-infrared light, using long chain cyanine dyes, see, for example, Mees and James, The Theory of the Photographic Process, 3rd Edition, MacMillan, 1966, pp. 198-201 and references cited therein.
It has been found that NIR sensitised photographic films, especially those having silver halide grains of mean diameter less than 0.4 micron, when supported by the edges in a glassless holder, to prevent contact with other surfaces, and given a uniform overall exposure from a laser diode scanner system emitting at 820 nm, produce images covered with broad swirling interference patterns, referred to hereinafter as "non-contact scanner fringes". These fringes are believed to arise as a result of the reflection of the exposing light from the two interfaces of the film element with surrounding air. The path difference between the rays reflected from the top surface of the film and the bottom surface is controlled by the thickness of the film at a given point, and the net phase difference causes either destructive or constructive interference, causing either diminished or increased exposure to be transmitted into the light sensitive emulsion layer at that given point. The fringes therefore follow contours of microscopic thickness variation in the film element itself, and cover the whole of the image area with broad lines usually about 1 mm apart and often several centimeters in length.
Non-contact interference fringes have not previously been reported in the literature in relation to silver halide emulsion materials. This phenomenon does not occur under the normal conditions of exposure with visible light because the turbidity of the photosensitive emulsion layer is sufficient to scatter the reflections from the back of the film element. However, because of its longer wavelength, infrared light is able to pass without serious scattering through small-grained photographic emulsions, and the coherence of the laser diode output enhances the tendency to form interference patterns. Thus, a photographic emulsion having silver halide grains of mean diameter 0.28 micron, with a coating weight of silver of 3 g/m.sup.2, shows detectable fringes. Lowering the grain size to 0.23 micron or reducing the coating weight causes more noticeable patterns, whilst emulsions of mean grain diameter 0.20 micron or less exhibit severe fringes after non-contact laser diode scanning.
Non-contact scanner fringes seriously degrade the quality of scanner images, especially those having continuous tone gradation. They are not only aesthetically displeasing but they also obscure important information conveyed by small density differences in the image. It is desirable to be able to use photographic emulsions having grains of mean diameter less than 0.4 micron preferably less than 0.30 micron. Fine grain emulsions having a grain size of 0.4 micron or less are advantageous in permitting high spatial resolution, and in having high covering power, permitting a lower coating weight of silver to produce a given maximum optical density after development. Accordingly, photographic elements for use with laser diode scanning systems must be capable of suppressing non-contact interference fringes.
The phenomenon of interference fringes is not unknown in optical recording systems. When exposing shiny surfaced photographic films in contact with other shiny surfaces, e.g. glass supports, dot screens or contact printing negatives, a common problem is the occurrence in the developed image of closely spaced concentric fringe patterns, known as "Newton's rings", see, Encyclopedic Dictionary of Physics, J. Thewlis, Ed., Pergamon, London, 1961, p. 878. These fringes arise due to optical interference between reflections from the top surface of the film and the bottom surface of the contacting support; the size of the local air gap determines the path difference between these two sets of rays, and hence whether their phase difference gives rise to a light or dark fringe causing additional or diminished exposure to be transmitted into the emulsion layer. Newton's rings tend to form isolated areas of pattern, radiating concentrically from the points of contact during exposure, with a narrow fringe spacing which becomes progressively smaller towards the edge of each pattern. These are quite different in appearance to the broad swirling non-contact scanner fringes which cover the whole image area with broad lines usually about 1 mm apart and often several centimeters in length.
Methods are known in the art to prevent formation of Newton's rings. For example, it is known to incorporate matting particles in the outer surface of films. Examples of known matting particles include silica, poly-methyl methacrylate (PMMA), other polyvinyl compounds including copolymers, starch or inorganic salts. The density of matting coverage varies from a relatively small number (e.g. applied at less than 0.1 g/m.sup.2) of fairly large particles usually 5 to 10 micron in diameter as disclosed in U.S. Pat. Nos. 4,235,959, 4,022,622, 3,754,924 and 2,322,037, to a particle weight of greater than up to 1 g/m.sup.2 or 50% of the topcoat binders as disclosed in British Patent Specification Nos. 2 077 935 and 2 033 596 and U.S. Pat.
Nos. 3,507,678 and 2,992,101 utilizing smaller particle sizes.
Use of visible laser light as illumination for contact screen exposure of both emulsions produces particularly severe Newton's rings fringes. U.S. Pat. No. 4,343,873 discloses a photographic element designed to minimise such fringes which includes a light-scattering layer through which the light-sensitive layer is exposed to laser light. The light-scattering particles have a diameter of from 50 to 150% of the wavelength of the illuminating laser. The light scattering layer may be coated as an outer layer on the photographic element or beneath other layers.
It is also known to use matting agents in photographic elements for non-optical properties, such as resistance to adhesion, abrasion resistance, retouchability, good draw-down in vacuum frames, and reduced static effects. An example of the use of a matting agent is an infrared sensitive film is disclosed in U.S. Pat. No. 4,266,010 which describes an emulsion topcoat containing PMMA of size in the range 0.2 to 10 micron in an acid-processed gelatin binder, stating this to be suitable for all types of photographic materials including infrared films. A further example is disclosed in U.S. Pat. No. 3,695,888 which describes a photographic emulsion sensitised to infrared light by cyanine dyes with mesoalkylamino substituents and specific super-sensitisers, stating that such elements can contain matting agents such as starch, titanium dioxide, zinc oxide, silica, polymeric beads, including 1 to 4 micron beads of a methacrylic acid-methyl methacrylate copolymer disclosed in U.S. Pat. No. 2,992,101 and 1 to 20 micron poly-methyl methacrylate beads formed by emulsion polymerisation as disclosed in U.S. Pat. No. 2,701,245.
It has been found that the known types of layers used on photographic elements for suppressing Newton's rings do not prevent the formation of non-contact interference fringes for photographic elements having a photographic emulsion of fine grain size sensitised to the near-infrared.